Initialize monitor-section output button

When loading a session the monitor-connection is made before
the GUI is initialized, the output button did not reflect
the actual connection.

gtk2_ardour/monitor_section.cc

@@ -629,6 +629,7 @@ MonitorSection::set_session (Session* s)
 
 			if (_ui_initialized) {
 				update_processor_box ();
+				update_output_display ();
 			}
 
 			SYNCHRONIZE_TOGGLE_ACTION (ActionManager::get_toggle_action (X_("Monitor"), "UseMonitorSection"), true);